As a catalytic conversion system for removing NOx contained in engine exhaust gas, there is proposed an exhaust gas purification apparatus disclosed in Japanese Unexamined Patent Publication No. 2000-27627. This exhaust gas purification apparatus injection supplies liquid reducing agent in a necessary amount corresponding to engine operating conditions, into the exhaust gas upstream of a reduction catalyst which is arranged in the engine exhaust pipe, to thereby subject the NOx and the liquid reducing agent in the exhaust gas to a catalytic reduction reaction, and thus purification process the NOx into harmless constituents. Here, in the reduction reaction, ammonia which has good reactiveness with NOx is used, and for the liquid reducing agent, a urea aqueous solution which is hydrolyzed by the heat and the water vapor in the exhaust gas and produces ammonia, is used.
However, in the reducing agent container which stores the liquid reduction agent, in order to be able to demonstrate the function as an exhaust gas purification apparatus, at least a densitometer and a water level indicator for respectively detecting the concentration and the remaining amount of the liquid reducing agent, as well as a heat exchanger through which the engine coolant is circulated to prevent freezing of the liquid reducing agent under a low temperature condition such as during winter season and so on, must be respectively fitted, and also it is necessary to respectively form a supply port and a return port for the liquid reducing agent. At this stage, regarding the densitometer and the water level indicator, since periodic inspection and maintenance is necessary, there is a requirement to detachably fit these to the reducing agent container.
However, even if the densitometer and the water level indicator are detachably fitted to the reducing agent container, if their attachment and detachment requires a lot of time and labor, inspection and maintenance takes time, so that the cost for maintaining the function of the exhaust gas purification apparatus is likely to rise. Furthermore, if the interval of inspection and maintenance is lengthened in order to avoid an increase in cost required for inspection and maintenance, there is the likelihood of the engine being operated with insufficient function of the exhaust gas purification apparatus.